This invention relates to colour selective filters and is particularly concerned with such filters in the form of a holographic reflector. It is extremely difficult to produce a filter which produces only a relatively narrow band colour when illuminated by broad band light, such as white light. For many applications, conventional colour selective filters which rely on the selective absorption within the filter of unwanted colours are insufficiently precise. Improved results can, in principle be obtained by the use of colour selective filters which are constituted in the form of reflecting holograms--these are sometimes termed holographic reflectors. Whilst it has proved possible to produce a very small holographic reflector having an acceptable performance, the corresponding performance for relatively large area holographic reflectors has proved to be unsatisfactory.